1. Field of the Invention
The present invention relates to radar systems, and more particularly, systems and methods of employing a traffic collision avoidance system (TCAS) to provide a radar function for an unmanned aircraft system (UAS).
2. Description of the Related Art
An unmanned aerial vehicle (UAV) (known also as a remotely piloted vehicle (RPV) or an unmanned aircraft system (UAS)) is an aircraft that is flown by a pilot or a navigator (sometimes referred to as a Combat Systems Officer) without a human crew member on board the aircraft. There is a wide variety of UAV shapes, sizes, configurations and characteristics. Historically, UAVs were simple drones (remotely piloted aircraft), but autonomous control is increasingly being employed by UAVs. Typically, UAVs may be classified as one of two general types: (1) those that are controlled from a remote location and (2) those that fly autonomously based on pre-programmed flight plans using more complex dynamic automation systems.
One of the largest users of UAVs today is the government of the United States, and in particular, the U.S. military. Military UAVs currently perform reconnaissance missions, as well as attack missions. UAVs are also being used in a growing number of non-military or civilian applications, such as firefighting or nonmilitary security work, such as surveillance of pipelines. UAVs may be preferred for missions that are perhaps less well suited for manned aircraft, if, for example, the mission is comparatively dull, dirty, or dangerous.
The abbreviation UAV has been expanded in some cases to UAVS (unmanned-aircraft vehicle system). In the United States, the Federal Aviation Administration (FAA) has adopted the generic classification of unmanned aircraft system (UAS). UAS is also used by International Civil Aviation Organization (ICAO) and other government aviation regulatory organizations.
There is a need for UAS aircraft to be able to fly within the national airspace (NAS) without having to obtain special approval on a case-by-case basis. This requires the UAS aircraft to be able perform surveillance of the airspace and automatically maneuver during flight path conflict between the UAS aircraft and another airborne object or vehicle. Such airborne objects or vehicles can include, but are not limited to another aircraft, a helicopter, a parachutist, a hot air balloon, a glider or any other airborne object or vehicle.
Many aircraft are now equipped with transponders within controlled airspace, and a traffic collision avoidance system (TCAS) can interrogate on 1030 MHz and receive transponder replies on 1090 MHz, as per RTCA document DO-185B (which is incorporated by reference herein in its entirety). This is known in the field as TCAS secondary surveillance. However, there are other objects or vehicles, as described above, that do not possess transponders, i.e., are non-equipped, and cannot therefore provide replies to TCAS interrogations. It is thus desirable that UAS aircraft be able to detect such non-equipped objects or vehicles and avoid them.
Primary tracking radars that provide radio frequency (RF) radar pulse outputs and receive reflections off of non-equipped objects or vehicles exist and are used or proposed for use in UAS aircraft as a means for obtaining range and azimuth tracking information on non-equipped airborne objects or vehicles. This information can then be used in conjunction with other devices, such as electro-optical devices, to develop a range track from the primary tracking radar, an altitude track and a bearing rate track from the electro-optical devices, thereby enabling the UAS aircraft to avoid non-equipped airborne objects or vehicles. Additionally, U.S. Pat. No. 7,414,567 (which is incorporated in its entirety herein by reference) discloses an ADS-B radar that use modified transponder reply transmissions to provide primary tracking of non-equipped airborne objects or vehicles.
Existing methods, such as those set forth above, are disadvantageous in that they require use of a separate primary radar tracking device and an antenna, each of which add considerable weight to a UAS aircraft. Adding such weight to a UAS aircraft limits the payload weight that a UAS aircraft can carry, which may exclude the ability to carry on the UAS aircraft other devices and systems, such as a TCAS which may be needed to fly within the NAS without having to obtain special approval on a case-by-case basis.
U.S. Pat. No. 7,414,567 also requires new modulation to be added to the industry standard waveform for ADS-B 1090 MHz replies, as described in RTCA DO-260B (which is incorporated in its entirety herein by reference). Such new modulation would likely have to be approved for use by the FAA, the Federal Communications Commission (FCC) and other international bodies before the technique could be used, such approval processes typically being lengthy in duration. It would also likely need to be proven that any added modulation applied to the standard ADS-B waveforms does not cause any degradation in the standard decoding function of ADS-B waveforms. Moreover, all equipment fielded to date is not capable of providing the necessary new modulation waveforms to utilize the methods of U.S. Pat. No. 7,414,567. Another difficulty with U.S. Pat. No. 7,414,567 is that the waveform being used for the radar function is 120 microseconds long. The 120 microsecond transmission obscures reflections from other objects and vehicles for 120 microseconds, while the transmission is being sent. This creates a tracking dead zone of approximately 10 nautical miles. Since a range of interest for UAS aircraft to detect non-equipped airborne objects and vehicles may be from 0 to 5 (or 0 to 10) nautical miles from the UAS aircraft, the radar function disclosed by U.S. Pat. No. 7,414,567, having the noted blind spot of 0 to 10 nautical miles, is not viable for use on UAS aircraft.
Thus, a need exists for improved systems and methods for providing a radar function for a UAS aircraft.